In silver halide photography one or more radiation sensitive emulsion layers are coated on a support and image-wise exposed to electromagnetic radiation to produce a latent image in the silver halide emulsion layer(s). The latent image is converted to a viewable image upon subsequent chemical photoprocessing.
Roentgen discovered X-radiation by the inadvertent exposure of a silver halide photographic element to X-rays. In 1913 the Eastman Kodak Company introduced its first silver halide photographic element specifically intended to be exposed by X-radiation (that is, its first silver halide radiographic element).
The medical diagnostic value of radiographic imaging is widely accepted. Nevertheless, the desirability of limiting patient exposure to X-radiation has been appreciated from the inception of medical radiography. Silver halide radiographic elements are more responsive to longer wavelength electromagnetic radiation than to X-radiation.
Low X-radiation absorption by silver halide radiographic elements as compared to absorption of longer wavelength electromagnetic radiation led quickly to the use of fluorescent intensifying screens (hereinafter, radiographic phosphor panels) when the Patterson Screen Company in 1918 introduced matched intensifying screens for Kodak's first dual coated radiographic element.
A radiographic phosphor panel contains on a support a fluorescent phosphor layer that absorbs X-radiation and emits longer wavelength radiation to an adjacent radiographic element in an imagewise pattern corresponding to that of the X-radiation received.
Hence intensifying screens containing fluorescent substances are employed to increase the exposure of a photosensitive plate or film without increasing the X-ray exposure dose to the object of the radiograph. These screens are customarily arranged inside a cassette, so that each side of a silver halide film, emulsion-coated on one or both sides, after the cassette has been closed, is in intimate contact with an adjacent screen. In exposing the film the X-rays pass through one side of the cassette, through one entire intensifying (front) screen, through the light-sensitive silver halide film emulsion-coated on both sides and strike the fluorescent substances (phosphor particles) of the second (back) intensifying screen. This causes both screens to fluoresce and to emit fluorescent light into their adjacent silver halide emulsion layer, which is inherently sensitive or spectrally sensitized to the light emitted by the screens.
The commonly used fluorescent screens comprise a support and a layer of fluorescent particles dispersed in a coherent film-forming macromolecular binder medium. Conventional X-ray screens have protective topcoats comprising, for example, cellulose acetate or other polymeric materials that form a coherent layer on coating. These topcoats are often inadequate to shield the active layer from abrasion caused by the rapid exchange of the film in and out of cassettes or automatic changer systems. Scratches can also occur during periodic cleaning of the X-ray screens by laboratories technicians. Mechanical damage due to scratches and abrasion can result in surface defects leading to artifacts in the radiographs produced. A topcoat must also provide a barrier to the penetration of moisture, in the form of water vapor or liquid water, which would degrade the performance of the phosphor. Moisture penetration, commonly has the effect of causing the panel to either have reduced light output, requiring the use of increased x-ray dose to produce the same radiographic film density, or causing more localized dimmer areas as artifacts in resulting radiographs. In addition, the prior art topcoats tend to stain when accidentally contacted by processing fluids (e.g., developer and fixer) associated with the film development or when unprocessed film is placed in contact with a fluorescent screen which has been cleaned with water but not thoroughly dried. The failure of the topcoat shortens the useful life of the X-ray screen, and the staining may cause unwanted image areas to appear on the film during exposure. Further rapid exchange of radiographic film in the cassette can lead to air entrapment if enough time is not given for the air trapped between the phosphor screens and the film to be purged. Entrained air can lead to localized loss of image sharpness due to separation of the film from the screen surface. None of these defects can be tolerated in the medical X-ray area where a patient's life may depend on the results.
Many improvements to protective topcoats have been described in the art. U.S. Pat. No. 6,221,516 B1 describes a radiation image storage panel that has a phosphor layer which comprises a protective film. The protective film is a coated layer containing at least 30 percent by weight of a fluorine containing resin which is soluble in an organic solvent, such as a copolymer derived from a fluoro olefin and other copolymerizable monomer, polytetrafluoroethylene or modified polytetrafluoroethylene. The protective film prevents lowering of sensitivity even if the panel is repeatedly used. U.S. Pat. No. 4,491,620 describes a topcoat or abrasion layer useful for protecting an x-ray intensifying screen comprising a copolymer of a fluoro ester and methyl methacrylate. The topcoat is flexible, adhesive, and nonstaining and permits the use of the x-ray screen in the modern rapid changer systems. U.S. Pat. No. 4,983,848 describes x-ray intensifying screens that have an improved surface made by bonding a thin, clear, transparent, tough, flexible, dimensionally stable polyamides film thereon. Such screens display very low average dynamic coefficient of friction, very good resistance to wear and low static susceptibility which permits long-term use in both cassettes and rapid handling incurred in changer systems. U.S. Pat. No. 4,059,768 describes a fluorescent x-ray image intensifying screen comprising outer layer containing solid particular material protruding from a coherent film forming organic binder medium and having a static friction coefficient at room temperature not higher than 0.30 on steel. When solid particular material protrudes from a surface there exists the risk of removing the particles during cleaning or other abrasive encounter resulting in degradation of the surface for example the formation of glossy streaks where the solid particulates have been removed.